It is known that mordenite can be treated with aqueous solution of NaOH at a pH greater than 10.5, in order to remove silicon dioxide.
The concentration of the sodium hydroxide solution can be 0.1 to 15N. The temperature is 0.degree. to 100.degree. C., preferably 25.degree. to 75.degree. C., and the treatment time is 5 minutes to 5 hours. This treatment increases the accessibility of the inner pores (see U.S. Pat. No. 3,326,797). These zeolites treated in this manner are used as hydrocracking catalysts or isomerization catalysts.